1. Field of the Invention
This invention relates to positioning of sheets in an active registration system, and more particularly, to pre-registering of sheets before they reach the active registration system.
2. Description of Related Art
Sheet registration systems deliver sheets of all kinds to a specified position and angle for a subsequent function within a printer/copier. Functions could include transferring an image to a sheet, stacking the sheet, slitting the sheet, etc. Conventional registration system corrects for skew and lateral offset. Skew contributors may be sheet supply angle, skew induced when the sheet is acquired to the feeder, inboard-outboard drive roller velocity differences on a common drive shaft. Lateral offset may be due to sheet supply location and sheet drive direction error. Sheet drive direction error is caused by the sheet drive shafts not being perpendicular to the intended sheet drive direction. This is a result of tolerances and excess clearance between: drive shafts and frames, sheet transport mounting features and machine frames and machine module to module mounting. In present day high speed copier/printers, active registration systems are used to register the sheets accurately.
In printers/copiers where an active registration system is used, a sheet is passed over sensor arrays from which the sheet skew and lateral or cross process offset is calculated. At this point the sheet is steered into the proper position by rotating inboard and outboard nip rollers at different velocities. This function must be performed in a specific time and distance. As the sheet needs to be moved faster and faster to increase overall productivity, the time to register the sheet to correct for skew and lateral offset decreases. With this, the acceleration and velocities at the registration nips increase to a point of failure.
One such active registration system is disclosed in U.S. Pat. No. 5,094,442 to David R. Kamprath et al., issued Mar. 10, 1992 that registers sheets in a feed path without the use of guides or gates. Laterally separated drive rolls are speed controlled to correct for skew mispositioning. Lateral registration is achieved by translation of the drive rolls transversely to the direction of sheet movement. Varying the speeds of the drive rollers equally controls longitudinal registration. The system reduces the required sheet path length to achieve correct registration, thereby allowing higher speed operation.
A method and apparatus for an active sheet registration is shown in U.S. Pat. No. 4,971,304 issued Nov. 20, 1990 to Robert M. Lofthus, which provides deskewing, and registration of sheets along a paper path in the X, Y and xcex8 directions. Sheet drivers are independently controllable to selectively provide differential and non-differential driving of the sheet in accordance with the position of the sheet as sensed by an array of at least three sensors. The sheet is driven non-differently until the initial random skew of the sheet is measured. The sheet is then driven differentially to correct the measured skew, and to induce a known skew. The sheet is then driven non-differentially until a side edge is detected, whereupon the sheet is driven differentially to compensate for the known skew. Upon final deskewing, the sheet is driven non-differentially outwardly from the deskewing and registration arrangement. A fourth sensor may be provided to measure the position of the sheet after registration with respect to desired machine timing.
U.S. Pat. No. 5,278,624 issued Jan. 11, 1994 to David R. Kamprath et al. shows a registration system for copy sheets that uses a pair of drive rolls and drive system for commonly driving both drive rolls. A differential drive mechanism is provided for changing the relative angular position of one of the rolls with respect to the other roll to deskew the copy sheet. A control system is supplied with inputs representative of the skew of the copy sheet and controls the differential drive mechanism to deskew the copy sheet.
A lateral sheet pre-registration device is shown in U.S. Pat. No. 5,697,609 issued Dec. 16, 1997 that includes a steerable pair of drive nips located in the paper path of an electrophotographic printing machine. A lead edge sensor detects when a sheet is within the steerable drive nips. The steerable nips are turned so that the sheet is transported toward a side registration sensor located in the paper path. When the side registration sensor detects the edge of the sheet an actuator causes the steerable nips to be straightened. The sheet may be forwarded to a second, higher accuracy registration device for final registration.
Even though the above-mentioned registration and pre-registration systems are useful, there is still a need to move sheets faster and faster to increase overall productivity, but the time to register the sheets to correct for skew and lateral offset decreases. With this, the acceleration and the velocities at the registration nip increase to a point of failure. One way to increase the latitude of the active registration system and decrease nip failure is to decrease the lateral offset.
Accordingly, pursuant to the features of the present invention, a method and apparatus is disclosed that answers the above-mentioned problem by providing an active pre-registration system that includes a pivoting and translating sheet supply elevator that is positioned by a stepper motor which is actuated in response to registration sensors to direct sheets to the proper lateral position. This will reduce, if not eliminate, the lateral offset and reduce the time and acceleration required registering the sheets.
This and other features and advantages of the invention are described in or apparent from the following detailed description on the exemplary embodiments.